When you took off the cpu fan did you also take off the heatsink? If you did really you should have cleaned off the thermal paste and applied some new. If you didnt then it could be you've disturbed the heatsink from the cpu, what im thinking is your cpu is now getting too hot and your system is shutting down to prevent damage.

tekgun you might be in training but you are a legend! I think you have hit the nail right on the head! AHH the old thermal paste got me! Might seem like a silley question...were do you but thermal paste...a specialist computer store I assume? Thanks again in advance.

i get mine online but any decent PC shop should supply it, you only need a small amount about the size of a grain of rice spread evenly over the cpu, arctic silver is the best do a little research with google just to go sure, and make sure to clean off the old paste first with some rubbing alcohol or similar.